A casket is a coffin, or a box in which the body of a dead person is buried. Some funeral traditions include a chance for mourners to see the deceased in the casket before burial.
Most caskets are made of wood — some plain, and others more ornamental. Many caskets are buried in cemeteries, although others are cremated, or burned to ash, depending on religious and personal preferences. You can also use the word casket to mean "small box," particularly a box for jewelry or other precious items, and this is the word's earliest meaning, from the Old French cassette, a diminutive form of case.